Gas analysers of the selective radiation absorption type with a calibration cell

ABSTRACT

A gas analyser of the infra-red radiation absorption type, particularly for measuring the amounts of CO 2  and CO in vehicle exhaust gases, comprises a pair of infra-red radiation sources arranged to direct their radiation along a measurement path and a reference path respectively, the radiation entering each path being periodically interrupted by a rotating commutator. Two measurement chambers for receiving the gas to be analysed and respective detection chambers for CO 2  and CO are alternately disposed along the measurement path, while two reference chambers and two more such detection chambers are alternately disposed along the reference path, corresponding detection chambers being connected to respective differential pressure sensors of the variable capacitance type. The analyser includes a calibration cell containing predetermined concentrations of CO 2  and CO. The cell is selectively interposable in the measurement path, and is connected to a flexible-wall chamber. The flexible-wall chamber defines with the cell a sealed enclosure, and serves to maintain the pressure within the sealed enclosure equal to ambient pressure.

The present invention relates to gas analysers of the selectiveradiation absorption type, such as those using infra-red radiation, andis more particularly concerned with improvements in calibration devicesfor such apparatus for compensating the influence of variations ofpressure and temperature on their indications.

It is known that analysers of the infra-red type generally comprise asource of radiation, an optical commutator for periodically directingthe radiation along a measurement path and along a reference path, and ameasurement detector constituted by a cell sensitive to differentialpressure variations and comprising a variable condenser microphone ofwhich the separation of the electrodes is modulated, at the frequency ofthe commutator, as a function of the pressure differences to which it issubjected. The voltage thus obtained at the output of the detector is,after amplification, demodulation and filtering, representative of theamount of the gas to be measured, which amount is displayed on anindicator.

This type of apparatus requires periodic recalibration in consequence ofthe drift with time in parameters of its various components, such as thesource and the detector, and the inevitable presence of dirt on theoptical paths, etc.

The problem of recalibration is easily resolved in the laboratory bymeans of bottles of calibration gas: a bottle of neutral gas, forexample nitrogen, permits the zero of the indicator of the apparatus tobe adjusted by filling the measurement container with this neutral gas,and a bottle of the gas whose amount is to be measured permits the scaleadjustment to be made by introducing a mixture of given concentration ofthis gas whose amount is to be measured in the measurement container andby adjusting the gain of the amplifier in order that the indicatordisplays this given concentration.

However, these calibration operations by means of bottles of calibrationgas are delicate and hardly practicable for carrying out bynon-specialised personnel, as is the case for example for motormechanics responsible for checking the amounts of the oxides CO and CO₂in the exhaust gas of vehicles, during the adjustment of theircarburators in order to meet regulations for limiting pollution in thisdomain.

The method of calibration thus consists in practice of utilising air asthe neutral gas for the zero adjustment, and of replacing the gas ofknown concentration supplied from a bottle for the scale adjustment by ashutter or a metallic grill which simulates only very crudely thephenomenon of selective absorption.

To the errors due to this imperfect method of calibration are addedother causes of error, resulting from variations of pressure andtemperature of the gas to be measured, for the number of molecules ofthis gas which are contained in the measurement chamber (which is offixed dimensions) and influencing the transmission of the radiationtowards the detector, is obviously a function of these physicalparameters. The following observations have been made.

(a) At constant pressure, the effect of variations of temperature on thegas introduces a measurement error of about ±6.5% (while the tolerancefixed by the standards is ±2.7%): this requires a compensation of theeffect of temperature, or a thermostatic control bringing the gas to bemeasured to a reference temperature.

(b) At constant temperature, the effect of the variation of atmosphericpressure can be even more important. In a given place, extremevariations of atmospheric pressure of ±50 millibars result in an errorof about ±5% (hence the necessity of regulating the pressure of the gasto be measured in certain cases). Similarly, the indication of theapparatus as a function of the altitude of the place of use can alsovary by significant amounts. For example, between sea level and 2000meters in altitude, which corresponds to a pressure variation of 220millibars, the value displayed on the indicator can be in error by 22%in the case of the amount of CO and CO₂.

To overcome these errors, apparatus in accordance with preferredembodiments of the invention is provided with novel means forcalibration, which take account of external conditions of pressure andtemperature, whatever they are.

According to the present invention, a gas analyser of the selectiveradiation absorption comprises: at least one source of radiation;

a measurement chamber for receiving a gaseous mixture containing atleast one gas whose concentration is to be measured, said measurementchamber being disposed in a measurement path to receive radiation fromthe source;

means for periodically interrupting the radiation entering saidmeasurement path;

at least one detector sensitive to, and disposed to receive, radiationwhich has passed through the measurement chamber; and

a calibration cell for cooperating with the said path, said cellenclosing a given concentration of the gas whose concentration is to bemeasured in a sealed enclosure.

Thanks to this arrangement, the influence of pressure can be taken intoaccount during the calibration of the apparatus, since the calibrationcell, by its construction, can readily be arranged to take the samepressure as the ambient atmospheric pressure. As far as temperature isconcerned, its influence can be neutralised in several ways: for exampleby bringing the gas to be analysed to a substantially constanttemperature at which the calibration cell is maintained, or by leavingthe calibration cell at ambient temperature during the calibration, andby bringing the gas to be analysed, under the normal operatingconditions of the apparatus, to this temperature. The second solution,which appears more simple, can easily be put into practice by making thegas to be analysed follow a sufficiently long path to have time to coolto, or to reach, ambient temperature.

A particular embodiment of the invention, applied to measuring theamount of gases CO₂ and CO present in the exhaust gases of vehicles,will now be described, by way of non-limiting example only, withreference to the attached drawing, which is a somewhat schematicrepresentation of an analyser according to the invention in perspectiveview. However, it is to be understood that the invention is not limitedto this sole embodiment, and that it can be applied to measuring theamount of a single gas other than CO or CO₂.

The analyser shown in the drawing comprises a measurement path I and areference path II. The path I comprises an infra-red radiation source 10and successively in the path of radiation, a measurement chamber 11 forCO₂ provided with inlet and outlet pipes 12 and 13 for the gases to beanalysed, a detector 14 for CO₂, a second measurement chamber 15 for CO,also provided with inlet and outlet pipes 13 and 16, and a detector 17for CO.

Symmetrically, the reference path II comprises analogous elements: asource 20, a reference chamber 21, a detector 24 for CO₂, a secondreference chamber 25 and a detector 27 for CO. The reference chambers 21and 25, of the same thickness as the chambers 11 and 15, are filled witha gas neutral to the radiation, such as nitrogen. The detectors 14 and24 for CO₂ are two chambers filled with CO₂ and respectivelycommunicating with the two halves of a chamber 31 constituting a wellknown device sensitive to variations of differential pressure. The twohalves of the chamber 31 are separated by a flexible membrane formingone of the electrodes of a variable condenser, the second electrodebeing fixed, so that pressure differences applied to the membrane areconverted into corresponding electrical signals. Similarly, thedetectors 17 and 27 for CO are two chambers filled with CO andcommunicating with the two halves of another chamber 32 analogous to thechamber 31.

The analyser also includes an optical modulator comprising a fixed disc33 having two apertures 34, 35 aligned with the sources 10, 20respectively, and a rotor 36 in the shape of a double fan driven torotate around an axis of symmetry 37 by an electric motor (not shown),for example at a speed of the order of 1500 turns a minute. Thismodulator is interposed between the sources 10, 20 and the chambers 11,21 and it permits the respective radiation of these sources to beperiodically interrupted on the paths I and II, for example at thefrequency of 50 Hz.

The electrodes of the condensers of the chambers 31 and 32 arerespectively connected to the inputs of two electronic processing chains38, 39 in order to amplify, demodulate and filter the measurementsignals produced thereby and to display the results on the CO₂ indicator40 and the CO indicator 41.

According to the invention, the analyser further comprises a calibrationcell 42, filled with the gas to be measured in known proportions. Thecell 42 is constituted by a circular chamber closed at its ends by twowindows of material transparent to the radiation, for example offluoride, which are mounted parallel to each other in planesperpendicular to the direction of the radiation. The cell 42 is incommunication with a flexible-wall enclosure 43, such as for example abellows, and as a result, the calibration gas enclosed in the cell 42can expand freely as a function of ambient temperature and pressure. Inthe example shown, the cell 42 is mounted on an arm 44 which alsocarries a fluoride disc 45 of thickness equivalent to that of the twowindows closing the cell 42, and this arm 44 can pivot around the axisof a shaft 46, under manual or electric control, and thus take twopositions: a first, or calibration, position in which the cell 42 isinterposed in one of the two optical paths, preferably the measurementpath I; and a second, or measurement, position in which the disc 45 issubstituted in place of the cell 42 in the same optical path to restorethe attenuation provided by the fluoride windows of the cell 42. Theenclosure 43 is for example mounted on the shaft 46 and is connected tothe cell 42 by an internal conduit 47 provided in the shaft 46 and thearm 44. In order that the two optical paths are balanced, an equivalentthickness of fluoride is also interposed in the orifice 35 on thereference path II. A calibration operation is effected in the followingmanner.

(a) Zero adjustment. Assuming that the two paths I and II are opticallybalanced, the measurement chambers 11 and 15 are filled with neutralgas, or in the absence of this with pure air with negligible amounts ofCO₂ and CO, and the zero adjustment of the two indicators 40 and 41, forCO₂ and CO respectively, is effected in the usual manner. The arm 44 isin this case in the measurement position (as shown in the drawing) inorder to present in the optical path I the same thickness of fluoride asin the reference path II.

(b) Scale adjustment. Assuming for example that in the analyser of COand CO₂ under consideration, the measurement chambers 11 and 15 for CO₂and CO have thicknesses of 1 millimeter and 5 millimeters in the opticalpath of the radiation respectively, and that it is desired to adjust thetwo scale points representing 10.5% of CO₂ and 4.5% of CO, thecalibration cell is filled with a mixture of calibration gas containing10.5% of CO₂, 27% of CO (since the radiation must traverse 6 millimetersof chamber before reaching the CO detector 17 in normal operation), andthe remainder nitrogen or another neutral gas. The measurement chambers11 and 15 are filled with neutral gas, or in the absence of this withpure air, the arm 44 is placed in the calibration position, so as tointerpose the cell 42 in the optical path I. The respective gains of theamplifiers of the channels 38 and 39 are then adjusted in order todisplay on the indicators 40 and 41 10.5% of CO.sub. 2 and 4.5% of COrespectively.

The calibration thus effected takes account both of the pressure andtemperature to which the analyser, and more precisely the enclosure 43(whose volume can be larger than that of the cell 42), is subject. Itsposition in the path of the gases to be analysed can also be chosen toassure a better equilibrium of temperatures during the calibration.

What is claimed is:
 1. A gas analyser of the selective radiationabsorption type, comprising:at least one source of radiation; ameasurement chamber for receiving a gaseous mixture containing at leastone gas whose concentration is to be measured, said measurement chamberbeing disposed in a measurement path to receive radiation from thesource; means for periodically interrupting the radiation entering saidmeasurement path; at least one detector sensitive to, and disposed toreceive, radiation which has passed through the measurement chamber; acalibration cell for cooperating with the said path, said cell enclosinga given concentration of the gas whose concentration is to be measuredin a sealed enclosure; and a flexible-wall chamber, forming part of saidsealed enclosure, for controlling the pressure in the interior of saidenclosure in accordance with the ambient temperature and pressure.
 2. Ananalyser according to claim 1, further comprising cooperating means forselectively interposing said calibration cell in said measurement path.3. An analyser according to claim 2, wherein said means for selectivelyinterposing said calibration cell in said measurement path comprise apivotable arms which supports said cell.
 4. An analyser according toclaim 1, wherein said calibration cell is closed along the path of theradiation by two windows of material transparent to said radiation. 5.An analyser according to claim 3, wherein said calibration cell isclosed along the path of the radiation by two windows of materialtransparent to said radiation, and said arm carries a compensationwindow of a transparent material identical to that of the windows of thesaid cell, and of the same thickness, said compensation window beinginterposed in the measurement path in the normal operating position ofthe analyser.